Equilibrium sampling through membrane based on a single hollow fibre for determination of drug–protein binding and free drug concentration in plasma

The determination of drug–protein binding and free drug concentration in plasma applying the equilibrium sampling through membrane (ESTM) technique has been studied using supported liquid membrane extraction in a single hollow fibre without any membrane carrier. In the extraction setup, the donor phase (plasma or buffer) was placed in the vial, into which was immersed the hollow fibre with the acceptor phase situated in the lumen. This proposed technique was applied to study the drug–protein binding of five local anaesthetics and two antidepressants as model substances, and the influence of the total drug concentration on the drug–protein binding was investigated. The brief theoretical background for determination of the drug–protein binding under equilibrium conditions is described. The developed method shows a new, improved and simple procedure for determination of free drug concentration in plasma and extent of drug–protein binding.